This invention relates to the supply of power to the trackway conductors forming the primary circuit of an inductive power supply system; in particular this invention relates to the supply of power to the trackway conductors connected in a series resonant circuit which comprises the primary circuit of an inductive power supply system capable of distributing inductive power over a distance.
An inductive power transfer system generally comprises (a) a generator of alternating current, (b) one or more conductors carrying the alternating current as tracks (the xe2x80x9cprimary inductive pathwayxe2x80x9d) to one or more consumers, (c) a space carrying an alternating magnetic flux as a consequence of current flow within the conductor(s), (d) a pickup means (the xe2x80x9csecondary windingxe2x80x9d), which intercepts a part of the magnetic flux, and (e) power consuming means. Systems originating from our developments have been characterised by inclusion of: (f) an operating frequency generally above 10 kHz, (g) a tuned, resonant primary and a tuned, resonant secondary, (h) loose coupling between primary and secondary, and (i) provide power control by means of partial decoupling of the secondary without significantly affecting other consumers sharing the same primary conductors.
We first developed parallel tuned primary inductive pathways for inductive power transfer. When inductive power transfer is to be applied to systems including relatively long primary inductive pathways or tracks, high-frequency alternating currents at the high voltage that is required to drive parallel-tuned tracks become rather difficult to generate in a practicable manner. The high voltage is required in order to insert sufficient current into the track, given an unavoidable track inductance. It is difficult to drive a long, parallel-tuned track because (apart from safety and insulation considerations) there are finite limits to the voltage that can be switched, and handled, using known technology. Furthermore, safety is a real issue. Track extension methods are not suitable for parallel tuned tracks because when the track length is increased the xe2x80x9cgroup velocityxe2x80x9d of the track tends to zero while the characteristic impedance at the required frequency tends to infinity. In contrast, series compensated tracks can be extended using xe2x80x9cmodulesxe2x80x9d including lumped series capacitors, without adding to the number of system poles.
Previous techniques for generating AC resonant currents in inductive power transfer have generally been optimised for the case of parallel-tuned circuits. An example high-frequency converter (see FIG. 1) has a half-bridge configuration. The action of the switches can be regarded simply as adding boosts to the resonating current, like maintaining a child""s movements on a swing. Usefully, the switches generally change state at or very close to a zero-voltage condition and switching losses are thereby minimised. In our preferred version, the resonating capacitance is generally a lumped capacitance within or close to the power supply, and the track inductance comprises most of the resonating inductance. The current in the track is a relatively pure sine wave. This type of converter is described for example in our Application No: PCT/GB92/01463. It can be regarded as a xe2x80x9chalf-bridgexe2x80x9d converter, where a centre-tapped inductor having a high reactance forms a passive half of a bridge and provides a DC return path. This type of converter is generally used in a xe2x80x9cslavexe2x80x9d mode in that its switching frequency is determined by the actual resonant frequency of the system. The particular converter that we have selected is quite efficient, has low switching losses, and the switches themselves do not carry the magnetising current of the track. However it does have a propensity to detuning and having an unstable frequency under heavy loads, and the energy stored in the DC inductor may cause a voltage and/or current overshoot if the load is reduced suddenly.
Now that the advantages of series compensated tracks (as defined later) are becoming evident, and series compensated tracks are seen as being more relevant to commercialisation of inductive power transfer, other types of power supply may be preferable.
Furthermore, for possible applications such as the provision of inductive power to rail vehicles, there is a need to raise the amount of current that can be fed into a given track, and to raise the distance over which that current can be effective, and in general there is a need for reduction of the capital costs associated with an installation.
A series compensated track for use in inductive power transfer systems may be described as an elongated conductive loop wherein the outgoing and return conductors are parallel and spaced apart. During installation the inductive reactance is maintained at or near a relatively small value despite extensions of length, by inserting sets of discrete capacitors in series with the conductors at one or more evenly spaced sites. One example xe2x80x9csmall valuexe2x80x9d of inductance is 620 xcexcHxe2x80x94in relation to a track energised at 15 kHz, resistance 0.4 ohms, circulating current 250 A, at 1 kV rms.
It is an object of this invention to provide improved means for energising an inductive power transfer system, or at least to provide the public with a useful choice.
In a first broad aspect the invention provides apparatus for the supply of power from an alternating power source to the primary conductors of an inductive power transfer system having at least one tuned primary trackway; each primary trackway having an inherent resonant frequency, wherein the apparatus includes at least one reactive network having an input and an output; the network being capable of being connected between the power source and at least one primary trackway, said network including at least one inductance having a selected value and connected to at least one capacitance having a selected value, said network being capable of affecting a transfer of power between the supply of power and the at least one primary trackway.
Preferably the tuned primary trackway includes one or more series compensating capacitors along its length
In a related aspect the invention provides apparatus as previously described, wherein the network comprises a transconductance pi network (as herein defined) including one or more inductors and one or more capacitors, the transconductance pi network having an input and an output; the network being capable of being connected at the input to the power source and at the output to at least one primary trackway, wherein the network is capable of providing a deliberately mismatched coupling between the source of alternating power and the primary trackway so that the source appears to present a substantially infinite impedance and the primary trackway appears to present a substantially zero impedance; and so that the voltage at the output of the transconductance pi network may in use be substantially greater than that provided to the input of the transconductance pi network.
In another related aspect the invention provides apparatus as previously described, wherein the component values within any one transconductance pi network are selected so that the network is capable of determining the proportion of circulating current to be fed from the output of the network into the primary trackway and so that the amount of circulating current within the primary trackway can be controlled.
In a further related aspect the invention provides apparatus as previously described, wherein the apparatus comprises at least one transconductance pi network, each network being connected to a corresponding primary trackway, the network or networks being capable by means of a selection of suitable component values of determining the proportion of circulating current that may be fed from the network into each primary trackway so that the amount of circulating current within the primary trackway can be controlled.
In yet another related aspect the invention provides apparatus as previously described, wherein the apparatus comprises more than one transconductance pi network each connected through controllable switching means to one primary trackway, so that the amount of circulating current within the primary trackway can be controlled by connection of the outputs of one or more networks at any time to the primary trackway.
In a still further a related aspect the invention provides apparatus as previously described, wherein the apparatus comprises at least one transconductance pi network connected to each primary trackway, the at least one network being provided with at least one set of additional reactive components capable as a group of being controllably switched into or out of the network circuit from a first set of components capable of determining a first current, to a second set of components, capable of determining a second current, so that the circulating current to be supplied from the output of the network into each primary trackway can be controlled from time to time and so that the amount of circulating current within the primary trackway can be controlled.
In a yet further related aspect the invention provides apparatus as previously described, wherein the apparatus provides switching means across the primary conductors of a primary trackway at a position where any one transconductance pi network is connected to the trackway; the switching means being capable when in a closed state of shorting the trackway so that the amount of circulating current within the primary trackway falls, and so that the primary trackway is in effect disconnected from the power supply yet the power supply itself does not experience a short-circuited output.
In an even further related aspect the invention provides apparatus as previously described, wherein one or more of the inductive components of any one transconductance pi network are selected so that the one or more inductive components are capable of entering a state of saturation if a current greater than an intended current should flow through the saturable inductor, so that the maximum amount of circulating current within the primary trackway can be limited.
In an even yet further related aspect the invention provides apparatus as previously described, having more than one primary trackway wherein each primary trackway forms a part of an extended primary trackway; an end of each primary trackway meeting an end of at least one other primary trackway near an alternating power source; each primary trackway being fed at the connection point by a separate transconductance pi network having a switching means connected across the output of the transconductance pi network, so that in use any primary trackway may be energised in response to the presence of a consumer of inductive power upon that primary trackway by opening the corresponding switching means, so that the power supply output may be directed down a selected section of the primary trackway and limitations of trackway length based on power supply capacity are substantially overcome.
In one more related aspect the invention provides an inductor suitable for use with apparatus including a transconductance pi network as previously described, wherein the inductor is provided with a ferrimagnetic or ferromagnetic core capable of saturation at an intended level of current, so that the total circulating current within the network may be limited.
In a still yet further related aspect the invention provides an inductor suitable for use with a transconductance pi network as previously described, wherein more than one turn of litz wire is held within a conductive container and apart from the inner surface of the container.
In a subsidiary aspect the invention provides inductor apparatus as previously described, wherein a ferrimagnetic core comprising one or more ferrite elements each having a xe2x80x9cCxe2x80x9d shaped or modified toroidal ferrite core including an air gap is threaded over about one turn of litz wire. In another subsidiary aspect the invention provides an inductor as previously described, wherein the one or more ferrite elements are each mounted by means of a support occupying the air gap.
In a second broad aspect the invention provides apparatus as previously described, wherein the apparatus further includes termination apparatus; the termination apparatus being connected between an end of one conductor and an end of the other conductor at a position remote from a connection to a power supply, the termination apparatus comprising in parallel connection (A) a substantially pure resistance capable of exhibiting the characteristic impedance of the track at substantially any frequency, and (B) a series tuned resonant circuit tuned to the inherent resonant frequency; the termination apparatus being capable at the resonant frequency of presenting a short circuit to the track and capable of presenting the characteristic impedance to the track at other frequencies, so that the harmonic content of the current circulating in the track is reduced.
In a third broad aspect the invention provides power supply apparatus for the supply of power to the primary conductors of an inductive power transfer system having at least one series-tuned primary trackway; each primary trackway having an inherent resonant frequency, wherein the power supply apparatus comprises a set of four switching means arranged in a first, and in a second series-connected pairs across a supply of electric power, the series compensated trackway being connected from the junction between the first series-connected pair of switching means to the junction between the second series-connected pair of switching means, the apparatus having control means capable of delivering switching commands capable of causing each switching means to repeatedly open or close in a cyclic relationship controlled with respect to the other switches, so that both switches of either pair are not closed at the same time and so that the series compensated trackway is repetitively connected across the supply of electric power first in one direction and then the other, at or near the inherent resonant frequency of the track.
In a related aspect the invention provides power supply apparatus as previously described, wherein the apparatus is driven from a voltage source of power.
In another related aspect the invention provides power supply apparatus as previously described, wherein the apparatus is driven from a current source of power.
In a further related aspect the invention provides power supply apparatus as previously described, wherein the apparatus is driven from a current source of power and further includes a parallel resonant circuit, capable of resonance at or near the inherent resonant frequency of the track, connected across the output of the power supply apparatus.
In yet another related aspect the invention provides power supply apparatus as previously described, wherein the apparatus includes control means capable of determining the controlled relationship; said control means being further capable of permitting variation of a pulse duration of the switching commands to at least one pair of switching means, so that the amount of current circulating in the series compensated track can be varied.
In a yet further aspect the invention provides power supply apparatus as previously described, wherein at least one of the switching means is adapted by means of installation of a capacitor across each switching means so as to be capable of being switched by a control means when in a zero-voltage state and by means of alteration of the inherent resonant frequency of the series compensated track to be slightly lower than the rate of delivery of control signals to the switching means.
In a still yet further aspect the invention provides power supply apparatus as previously described, wherein the power supply apparatus includes a parallel resonant circuit having a resonant frequency, and includes at least two switching means arranged to be capable of connecting a current from a current source to the resonant circuit and operated so that repetitive complementary closures of the switching means at a frequency close to the resonant frequency is capable of causing resonant current to flow within the parallel resonant circuit, and a power output from the power supply apparatus is connected across the parallel resonant circuit, wherein the power supply apparatus employs four switching means in a full bridge configuration to connect current from the current source to the resonant circuit.
In a fourth broad aspect the invention provides apparatus for supplying inductively transferable power over a distance; the apparatus comprising (1) a conductive primary pathway or track having a pair of conductors capable of carrying the current; the track including one or more sets of series capacitors capable of compensating for the series inductance and hence comprising a series of circuits capable of resonating at a resonant frequency, the track being terminated with (a) a short-circuit at the resonant frequency and (b) at the characteristic impedance of the track for other frequencies, and (2) a power supply capable of generating alternating current at a desired frequency and including an adaptation for feeding its output into the track.
In a related aspect the invention provides a first type of power supply apparatus for energising a conductive primary pathway or track of an inductive power transfer system with alternating current, wherein the power supply apparatus includes a set of four switches arranged in a bridge configuration and having a supply of electric power, and having means for causing each switch to open or close in a controlled relationship with respect to the other switches, so that the series compensated track, connected across the other arms of the bridge, is repetitively connected to the supply of electric power first in one direction and then the other, at or near the inherent resonant frequency of the track.
Preferably the controlled relationship permits variation of the phase of the drive to at least one pair of switches, so that the amount of current circulating in the series compensated track can be varied. Preferably any or each of the switches can be switched when in a zero-voltage state by a procedure comprising (a) installing a capacitor across each switch, and (b) supplying the switching commands at a frequency slightly higher than the natural resonant frequency of the system, including the series compensated track.
In a further related aspect the invention provides a second type of power supply apparatus for energising a conductive primary pathway or track as previously described of an inductive power transfer system with alternating current, wherein the power supply apparatus includes a parallel resonant circuit having a resonant frequency, a current source, and at least two switches arranged to connect the current to the resonant circuit and operated so that repetitive complementary closures of the switches at a frequency close to the resonant frequency is capable of causing resonant current to flow within the parallel resonant circuit, and a power output from the power supply apparatus is connected across the parallel resonant circuit.
In a still further related aspect the power supply apparatus employs four switches in a full bridge configuration to connect current from the current source to the resonant circuit.
In yet another related aspect the invention provides a method for removing the current from a primary inductive pathway fed from a trans-pi network, wherein the method comprises shorting the primary inductive pathway at or near the point of feed.
In a yet further related aspect the invention provides a method for varying the current circulating within a primary inductive pathway fed from more than one network, wherein the method comprises controllable addition of the outputs from each of the more than one network into the primary inductive pathway.